Automatic train-control system



feu 10, i925.

L c. s. BUSHNELL .VUTOMATIC TRAIN CONTROL SYSTEM Filed March 7, 1921 retentea Feu-1o, 1925. i,526,292

UNITED STATES PATENT OFFICE.

CHARLES S. BUSHNELL, OF ROCHESTER, NEW YORK, A SSIGNOR TO GENERAL RAIL- WAY SIGNAL COMPANY, 0F GATES, NEW YORK, A CORPORATION 0F NEW YORK.

AUTOMATIC TRAIN -CON TROL SYSTEM.

Application filed March 7, 1921. Serial No. 450,149.

To all wko/m. t may concern: than with the purpose of illustrating the Be it knownthat I, CHARLES S. BUSH- construction and arrangement of parts NELL, a citizen of the United States, residpreferably employed in practice;

ing at Rochester, in the count of Monroe Fig. 3 is a top plan View of the car-carand State ofNew York, have invented cerried Climent, showing its elongated and tain-new and useful Improvements in Autocurved side .pole pieces; and

matic Train-Control Systems, of which the Fig. 4 is a .top plan view, similar to Fig.

following is a specication. 3, showing a modiiied construction of the This invention relates to automatic train vextended side pole pieces. control systems for railroads, and more par- The present invention is a specificy form ticularly to an improved impulse transof the same generic invention disclosed and mitting means of the inductive type by claimed in the application of IV. K. Howe, which the desired controlling impulsesor Serial No. 287,132, filed Apr. 3, 1919, and influences may be communicatedor transin my prior application Ser. No. 407,302, mitted from the trackway to moving cars -iiled Sept. 1, 1920; and I desire to have it by magnetic induction through an interunderstood that none of the subject matter, vening ail--gap either broad or specific, disclosed in either One important'principle to be observed of said applications is claimed herein.

in the construction of safety devices or sys- The impulse transmitting means embodytems forrailroads, is'that of 'failure on the ing the present invention comprises in genside of safety. Following out this prineral a track element T, located at the deciple in an inductive type of train control sired control points along the track and system, it is expedient to utilize the inhersuitably controlled in accordance with ent magnetic properties of a body of iron traffic conditions; and a car element and on the track to produce the stopping effect, associated parts cooperating with said track so that if wires break, batteries become eX- elements so as to cause interruption of a hausted, or other similar failures occur, circuit on the car, or in some other suitthe safety of the system will not be deable way produce an action on the car under stroyed by such failures, but the train will dangerous traliic conditions which can be be stopped. When 'a trackway impulse deutilized in any of the well known ways to vice ofthis kind is employed, however, there regulate the movement of the car.

is introduced the problem of preventing Various arranglments of trackway cirimproper stopping of the train at the track cuits for control g the condition of the rails of crossing, Crossovers, and the like, track elements T may be employed, and in which possess the same magnetic qualities the accompanying drawings there is shown as the regular track element. The present only one typical arrangement. Retferrin invention is primarily directed toward proto Fig. 1, the track rails 1 arevdivided by viding a car element and associated parts insulated joints 2 into blocks in the usual ,which will be influenced to a minimum exway, one block I with the adjacent ends of tent by track rails extending crosswise of two other blocks H and J being shown. the running rails, and which will at the Since the parts associated wit` the various same time respond to the influence of the blocks are the same, for convenience the regular track element in its stopping conwill be given like reference characters with dition. vOther features and advantages of distinctive exponents. Each of the blocks is the invention will appear hereinafter as the provided with a track battery 3 and a track description progresses. rela 4, in the same way. as in ordinary In the accompanying drawing, Figs. 1 bloc signal systems. The -train control and 2 show diagrammatically the tracksystem may be used with or without the way and car equipment of a` system emusual fixed signals; and I have illustrated bodying this invention, the parts andcirsuch fixed signals S conventionally, without cuits being shown more with the view attempting to illustrate their well known of making it easy to understand the nature control circuits. and mode of-operation of the invention, The track element T in the specific form.

' vance, so that the coils 7 are open-circuited.

illustrated comprises a U-shaped core orl yoke 5 of magnetic material, which has enlarged pole pieces 6, and which is provided with coils 7 on its legs. These coils 7 are wound and connected together so that the E. M. FFS induced therein by a change in flux passing through the yoke 5- will be cumulative. A track element T is located at the appropriate control point at or near the entrance to each block, and as shown, a track element is placed at a short distance in the rear of the insulatedjoints 2 at the entrance to the block. These track elements are preferably disposed crosswise of the track between the track rails at one side of the center line of the track; but this particular construction and arrangement is not compulsory, and they nay be constructed differently and disposed lengthwise of the track or outside of either track rail, as found best adapted to particular working conditions, provided that the `construction and disposition of the car element hereinafter described is correspondingly changed to provide for the necessary cooperative relation. In the type of trackway circuits shown inv Fig. 1, the coils 7 of each track element T are connected in series in a circuit of low resistance through the front contact of a line relay 8. The energizing circuit for v this line relay 8 includes a front contact ofthe track relay 4 of the corresponding block (at the entrance to which thetrack element in question isA located), and a front contact of the track relay of the next block in adwhen either theicorresponding block or the next block in advance 1s occupied.

The car element, designated as a whole by the letter L, comprises a' magnetic yokejor core Y having three vertically disposed legs 9, 10 and 11. The spacing between legs 9 and 10 and 10 and 11 is the same as that between the legs of the track element T; and the car element is supported in any suitable manner `fromthe frame of the locomotive or other vehicle crosswise thereof, so.that two of its legs, as 9 and 10, pass directly `over the pole pieces of the track element T, as shown in Fig. 2. The manner in which the car element may be supported will vary according to the type of locomotive or car, andy it has not been attempted to' show any particular construction for thisl purpose. In the case4 of the ordinary locomotive a convenient place for the car element L is found under the pony truck; and in all cases it is desirable to keep the pole pieces of the car element as far away from any viron parts of the engine frame as possible. The car element may be supported so that it can be adjusted vertically, but it is considered preferable to attach the car element rigidly and non-adjustably in most instances, for the 'reason that such adjustment may not always aaaaaaa be properly made. The parts of the car element will in practice be enclosed and protected in a casing of suitable, non-magnetic material.

. The middle leg 10 of the car element isr formed with an enlarged pole piece 12,.'preferably slightly elongated. The outside legs 9 and 10 are provided with elongated or ex'- tended pole pieces 13 and 14, as best shown in Figs. 3 and 4, for the purpose hereinafter explained. 'These pole pieces 13 and 14 may be made substantially semi-circular, as shown in Fig. 3, or curved towards each other in a suitable arc.; or, as shown in Fig. 4, they may be made L-shape, with overlapping ends. The yoke Y and its extended pole pieces made up of laminations bolted together, but maybe made solid, or partlylaminated, as found expedient. The extended pole pieces 13-14 will in practice be braced and supported ina suitable manner; 'and while they may be substantiallyv uniform in cross section, as shown, under certain circumstances it may be preferable to make their cross section variable to adapt the magnetic reluctance to variations in the total flux.

The car element L and the track element T are disposed so as to make the air gap between their pole pieces as sho-rt as consistent with the roadway and equi ment clearances on fthe particular railroa ln one arrangement which has been found to work out satisfactorily in practice, the track element is located with the upper'faces of its pole pieces 6 approximately on a level with, or one inch below the top of the running1 .rails, and the car element is located 13-14 are preferably sQthat the under faces of its pole pieces 12,

13 and 14 are .approximately two'and onehalf inches above the track rails, when the wheels of the vehicle are worn to the smallest diameter.. If the car element is Jnonadjustably supported from the vehicle, its relation tothe runnin rails Awill vary as the diameter of the w eels, changes due to wear. The variation on account of-wheel wear ordinarily does not exceed one and one-half inch; and with a non-adjustable car element, the' total air-gap varies from a minimum of two and one-half inches to.

four inches. These particular figures are merely. illustrative, and other air-gapsmay be used. l i f On the two outside legs 9 and` 11 of the car element L are two coils 15 and a16,'conp veniently termed primary coils, which are wound with the same number of turns, and are connected to ether in an energizing circuit', soy that t ey will create magnetomotive-forces tending to sendY ilux' in the same direction in the back yoke Y `and outside legs 9.a.nd 11 and in the opposite direction through the middle leg 10. Putting it another way, if the coil 15 makes a north and reliable in construction.

vaverage potential of pole at its pole piece 13, the other coil 16 makes a south pole at its pole piece 14. On the middle leg 1Q is a coil or winding 17 conveniently termed the secondary coil. Thecoils 15 and 16 are, therefore, oppositelywound as far as their relation to the magnetic circuitthroughv the secondary coil 17 is concerned Since the impulse transmitting means embodying the present invention may be used to control any suitable type of brake or train control apparatus, and since the particular form of this apparatus forms no part of the present invention, I have illustrated conventionally a device K, in the form of an electro-pneumatic valve, which may be used to vent'the train pipe and apply the brakes directly; or govern the operation of any other form of speed regulating or brake controlling mechanism. This device K is normally energized, and is set into operation when deenergized. While the device K may be made so as to be directly responsive to the impulse, I consider it preferable in most cases to employ a control relay R forV governing said device K. This relay is of course Vconstructed to have the operating characteristics conforming With its use, but since these features form no part of the present invention, this relay has been illustrated conventionally.

While the'control relay R may also be made directly responsive to the impulse, it is considered preferable to employ an amplifying device responding to the impulse yfor governing said relay, in order that the working currents'for said relay may be larger and the relay made more substantial pose a device of the thermionic or vacuum type, commonly known as an audion, is employed. This audion, beingweli known in the art, is illustrated conventionally, and comprises 'a lilament F, a grid G, and a plate P. This audion is connected up witlr suitable sources of current, illustrated as batteries A, B and C, the secondary coil 17 of the car element, and the control relay-R, so that said relay is caused to open its contactswhen the car passes a track element T in the active stopping condition. These electrical connections may take various forms,

.and the arrangement shown` in Fig.'2 is merely typical.

Referring to Fig. 2, the lilament F of the audion isheated by the battery A.. The rid is connected in circuit Withthe secon arycoil 17 and the battery C, so that a positive potential with respect to' negative end or mally maintained.

Thisl permits current to flow inthe plate circuit through the relay R. The primary coils 15 and 16 are energized by a circuit which may be traced as follows Commencing at battery B,

For this purlthrough the the filament F is norwires 20 and 21, armature 22 of relay R, wire 23, train control'device K, Wires 24 and 25, stick contacts 26 of the device K, wires 27 and 28, primary coil 16, wire 29, primary coil 15, wire 30, adjustable resistance 31, wire 32 thro-ugh the battery C back to battery B. In order to protect the relay contacts from injury or deterioration by the arcs accompanying the interruption of the inductive circuit through the primary coils, a resistance 33 is preferably connected across these contacts. The stick contacts 26 are connected to the co-re or other moving part of the device K, and arel opened when said. device is deenergized. In order to permit restoration of the train control device K, after it has operated, a push button or similar reset switch O is provided to close a shunt around the stick contacts 26. The reset switch O may be enclosed in a locked case, may be accessible only from the ground, or may be protected in some other suitable way so that it can not be maliciously manipulated; but since these detail features have no particular bearing on the resent invention, no attempt has been mage to illustrate them.

Operation. When the car is traveling between the track elements T, the relay Ry is energized, and in turn maintains the train control device K energized. The current in the primary coils 15 and 16 tends to send iuX around through the yoke Y and across the space between the outsidev pole pieces 13 and 14. There is little, if any, flux passing through the middle leg 10, because the magneto-motive-torces through it are opposing and the magnetic circuits are constructed so as to be as nearly balanced as possible.

Assume now that the car element L passes over a track element T having its coils 7 open-circuited, corresponding to dangerous traffic conditions ahead, that is, when a stopping impulse shouid be transmitted. The track element in this` condition presents in eiiect what may be termed a dead magnetic loop for two legs of the yoke Y, these two legs being 9 and 10 in the arrangement shown; and this magnetic loop changes the reluctance of the partial magnetic circuit through the primary coil 15 and the secondary coil 17, decreasing the reluctance from a normal to a minimum and then back to a. normal as the car element passes over the track element. This change in reluctance causes a change in flux secondary coil 17, haps partly due to' diversion of iiux from leakage paths through the middle leg 10 and partly due to the increase in the total flux. This change in flux through which is of an alternating one cycle.

character, that is, This induced E. M, F., either per-f l on the first" or second half of the cycle, op

poses the voltage lof the' batter C,Ythereby reducing the grid potential, lan in turn, o n account of the well-'known characteristics of the audion, decreasing the current in the plate circuit through the relay R; and with the proper selection of values for the-variable factors to conform with the particular working conditions, the current through the relay R is thus reduced below the amount required to hold up its armature 21. It is observed here that the polarity of the E. M. F. induced in the secondary coil 417 will depcnduponl the direction of the changing flux through it, which in turn depends on the way in which the primary coil 15 is wound, and how it is connected with its energizing batteries Bwd It is evident that an arrangement may be set up so that the grid potentialI is decreased either on the first or second half of the cycle. My investigations to date would indicate that the best'results are obtained under ordinary circumstances by using the first half of the c cle.A

yWhen the armature 22 of the relay R drops, it interrupts the energizing circuit for the control device K, thereby setting said device into operation; and by virtue of the stick contacts 26, said device continues in operation' after the armature 22 again closes, until the reset switch O is closed. The 're-^ sistanc'e 33 is, of course, so proportioned that, when cut into the circuit through the i device K, it will reduce the current therein below the hold-up current for said device.

Suppose now' that traffic conditions are safe, and the coils 7 of the track element T are included in aclosed circuit of -low resistance. When the car element L passes over a track element in this clear or safety condition, the car equipment is. not influenced, atleast, not enough to cause operation of relay' RQ, This is explained by the fact that the coils 7 oppose or choke back the passage ofthe flux through yyoke 5, vthere y causing little change in flux through the secondary coil 17. It is'found in practice preferable to proportion the copper and number of turns in the coils 7 so as to obtain the-maximum choking action, with the small external 'resistance of a relayv contact or the like'in series therewith. Also, -these coils may extend the entire length of the core 5, or for any desired part thereof, so as to avoid leakage of flux around them.

This operation just described has many features in common with systems disclosed in' yprior applications, and it should be understood that many of thesefeatures are not claimed herein because they form the subject-matter of said prior applications.

While the various coils, batteries, etc., must be selected to conform to variations in working conditions, one arrangement which I have found satisfactory with ordinary airgaps consistsI of seventeen (17) inch spacmg between the legs 9-107-11; primary coils .15 and.16 of 3000 turns; secondary coil 17 18,000 turns; and batteries B and C 24 andV 10 volts respectively. With a relay operating upon approximately 50% dropaway as compared with its ick-up current, such a line-up of parts wi work satisfactorily through an air-gap lof four -or more inches.l n

From the foregoing it will be observed that the stopping impulse is produced by causing a source of M. M. F. on thecar (primary coils 15 or 16)4 to send flux through a 'car-carried receiving coil (secondary 17) .in response to a magnetic loop or bridge on qualities and larger cross sections of the latter, it is desirable to reduce the rail-effect as much as possible, without altering the effectiveness of the regular track elements, so as to obtain wider margins of operation 'and assure reliable performance under the varying conditions of temperature, speed, air-gap, battery voltage, and the like, which ing any particu ar adjustment.

@ne important feature of the present invention is the way in whichthis rail-effect v must be met in ractice, and without requir# is minimized. Assume that the car 'element L shown in Fig. 2 passes over a rail or simi-- lar magnetic body7 disposed at right angles to the running rails. Such a rall forms a magnetic bridge between the legs 10 and 11, as well as between the legs 9 and 10; and under these conditions, the change in flux in thesecondary coil 17 is very slight and practically negligible as compared with that occurring when-only the two legs 9 and 10 are bridged by a track element T. This result is attributed to the fact that the flux emanating from the outside primary coils 15e-16 follows the rail in question and does not pass through the middle leg 10 on which the secondary coil 17 is mounted. Ltshould be remembered that the outside primary legs 9 and 11 are of opposite magnetic polarity, and it is thought that the flux takes the ath from one primary pole piece through t e rail to the` other pole piece and back through the yoke Y, lnstead of pssing throu M. F.s 0%

the two rimary coils 15 and'l are acting toge er to send flux the middle leg 10. The

.tion above mentioned,

l at a sharp angle, that is,

through this rail circuit, whereas these M. M. F.s are opposing each other in the middle leg 10. In any event, it is found that .the arrangement shown and described maintains such a balancethat the grid potential is not materially affected by a rail spanning the two outside legs 9 and 11 of the car element' L.

Since the track rails over which the car element L will have to pass under practical conditions may he at various angles with respect to the running rails, it is necessary to make provision for the same balancing action above explained when the rail-is at an angle as, for example, when the car runs over the point of a switch or crossover. This is accomplished in accordance with the same principle outlined in my prior applicathat is, by elongating the pole pieces 13-14 of the two outside primary legs 9 and 11, so that a rail at any angle affords a magnetic connection between said outside legs before 'coming close enough to influence the secondary coil 17. These poleipieces 13-14 may be extended parallel with each other and with the running rails, the car element being disposed crosswise ofthe track, but to take care of rails rails almost parallel with the running rails, such parallel extended pole pieces would have to be so long as to beV .undesirable for practical use and for most cases I prefer to" use pole pieces curved toward eachother as shown in Fig. 3, or the L-shape pole pieces 13a-14l with overlapping ends shown in Fig. 4. The tips of the extended pole pieces 13-14 are preferably separated by an air-gap as shown, so as to maintain a reluctance between the outside legs 9 and 11 through said extended pole pieces which is relatively high as compared with the reluctance of the path through the track element. The length of this air-gap does not seem to be material, however, and under favorablel conditions, this air-gap may be omitted. l

When the car element L, equipped with such elongatedpole pieces 13 and 14, passes over a rail set at an angle with respect to the running rails, this rail will be under these extended pole pieces at the time it is passing under the middle leg 10, so that the iiux is balanced out of the seconda-ry 17 so to speak, and the rail effect is minimized in the same way as above describedv in the case of a rail at right angles to the running rails. It may be explained here that the difference in the length of iron path for the magnetic circuits under consideration does not seem to be material, because of the relatively large air-gaps in said circuits. The modified construction shown in Fig. 4 has the advantage of being more compact, and also provides what may be said to be a complete protecting enclosure for the secondary coil, it being impossible for a rail at any angle to pass under the secondary coilY 17 without at the same time bridging the pole pieces 13;L anad The elongated pole pieces 13 and 14 seem to affect the impulse received from the regular track element T to a very slight ex tent, and by the use of the three-legged element, with the-two primary coils, etc., as described, it is found that the margin between operation of the relay by rail effect and by track element is greately increased, so that the system will work satisfactorily throughout Athe range of the varying conditions found in practice, without requiring adjustment of the air-gap, resistance, battery voltage, or the like. This is an important advantage of the invention, inasmuch as it obviates the need for adjustments and thus eliminates the possibility of the devices being mis-adjusted in such a way as to produce a dangerous condition.

While I have described my invention in connection with a specific type of system, including an audion, the invention is more general in its application. The particular construction shown and described may be modified and adapted in many respects to suit the particular working conditions; and I desire to have it understood that the illustration and description herein does not exhaust the adaptations and modifications of the means and functions thereof constituting the invention.

What I claim is:-

1. A. car apparatus for automatic train control systems comprising a three-legged yoke of magnetic material, oppositely wound primary coils on the two outside legs of said yoke and disposed so that the legs traverse different paths during the progress of the train, a source of current energizing said coils, a secondary coil on the middle leg of the yoke, and means having an operating circuit connected to said secondary coil.

2. Carl apparatus for automatic train control systems according to claim 1 and including an audion having its grid circuit connected with said secondarycoil, and train control apparatus governed Vby the plate circuit of said audion.

3. Car apparatus for train control systems according to claim l, and comprising electro-responsive train control means included in the energizing circuit for said primary coils.

4. Car apparatus for train control systems according to claim 1, and comprising a three-legged yoke having extended pole,

ieces for the outer legs arranged so as to be bridged by a rail at any ang e while said rail is passing under the mi dle leg.

, 5. Car equlpment for automatic train control nsystems, comprising two opposing electro-responsive sources of magneto-motive-force, a secondary coil, a pair of partial magnetic circuits through said secondary coil and said respective sources, train control means responsive to a change of flux through the secondary coil, and a track element which completes .one but not both of said partial circuits.

6. Car equipment for automatic train control systems, comprising a secondary coil, two partial magnetic circuits disposed cross-Wise of the direction of movement of the train each passing through said coil, opposing sources of magneto-motive-orce in said circuits, and train control means responsive to a change in lux through said secondary coil.

7. Car-carried apparatus fory automatic train control systems comprising, an inductive influence receiving coil located so as to,

inductively communicate with suitable influence communicating devices located along the trackway; sociated with said relay closed only when said relay in its energized position;v a circuit including said coil, said. -relay and the contact lof said relay in series; a discharge resistance'connected in multiple with said contact; and an electrically operated device having a stick contact in series with its energizing circuit controlled by said relay for restricting the movement of the train.

8. In an automatic train control system, impulse transmitting means comprising a track element constituting a magnetic body when in the stopping condition, and la carcarried element p-rovided with balanced partial magnetic circuits each including a source otl magneto-motive-force, said sources acting upon a receiving secondary coil.

9. An impulse receiving device for railway vehicles comprising a car-carried threelegged yoke, means for magnetizing said yoke with opposite poles. at itsoutside legs, means respondin to achange in flux through the mid le leg, said outside legs having extended pole pieces, and inductive trackway means for bridging only the space between the in iddleleg and one ofthe outside legs at one time." A

l0. Impulse transmitting means for automatic train control systems, comprising, in combination, a three-legged magnetic yoke on" a vehicle, means for magnetizing said yoke with opposite poles atthe two outside legs thereof, means iniuenced by a change in iiux through the middle leg, a track element spanning the middle leg and one of the outside legs, and pole pieces extending from said outside legs substantially encircling said middle leg.

11. A car element for automatic train control systems of the inductive type coma relay; a contact asprising, a thrbe-legged yoke, the outside legs of said yoke having extended L-shaped crate magnetically with a magnetic b ody along the track, a receiving coil on said core operatively connected to ,said relay, and tivo opposing sources of magneto-motive-force both tending to send flux through the receiving coil.

13. A car element for automatic train control systems comprising a three-legged yoke, and pole pieces of general L-shape eX- tending horizontally from the two outside legs of said yoke in the same plane` and surrounding the third leg.

. 14.. In an automatic train control system, the combination of an influence receiving 'device on a vehicle comprising, a car element having a partial magnetic circuit, a pluralityof sources of magneto-motive-force in said circuit, a secondary coil, trackway ,means for controlling the passage of iuX from only one of said sources of magnetomotive-orce through the secondary coil, and means for 'detecting'a change of nuxthrough saidv secondary coil.

l5. In an automatic train control system, the-combination of an influence receiving device on a vehicle comprising, means 'constituting a partial magnetic circuit, a receiving coil magneticall associatedwith said circuit, a plurality o sources of magneto-motive-force in said circuit, and trackway means rendering one of said sources only effective to send flux through the receiving coil.

16. An impulse receiving device for railway vehicles comprising, a car-carried back yoke having depending legs to form two partial magnetic circuits, two sources of magneto-inotive-force each actin to produce a held of iux through one o said partial magnetic circuits, means for detecting the quantitative relation of the 'uxes through said partial magnetic ycircuits due to said sources of magneto-motive-force, and inagnetizable trackway means for bridging only one'oi' said partial magnetic circuits.

-17. An influence communicating device for automatic train control systems comprising', a car-carried back yoke of magnetic material having three projecting legs, two sources of magneto-motive-,force tending to send flux in opposite directions through one of said legs, means for detecting a change of ux through said leg, and traiic con-A trolled trackway means for increasing the effect ot only one of said sources neto motive-torce upon said means.

IS. lntiuence communicating means for automatic train control systems comprising, a secondary coil, two partial magnetic circuits linking said secondary coil and each having a source ot magneto-moti\'e-torce associated therewith. trackwayv means for of niagdetecting reducing the reluctance ot one of said magnetic circuits only thereinv causing a change ot tlux through said secondarnsY coil,` and means for detectingl such change of flux.

1f?. Intlucncc connnunicating means for automatic train control systems comprising, a -volte ot` magnetic material having three legs. a secondary coil on one leg, separate sources of niagneto-niotive-force associated with the other legs and acting upon said secondar)p coil. and tracliway means for di- 'tclting flux from only one ot said sources 21:. In an automatic train control systeem. the combination ot' a communicating device comprising two partial magnetic circuits cach haring an exciting coil, a normally energized train control device connected in a stick circuit in series with said coils, meansl governed h r a change ot flux in said magv netic circuits tor controlling said stick circuit, and manual means protected against malicious manipulation for restoring said train control device.

22. Influence communicating means for automatic train control systems comprising, a three-legged element of magnetic material, a coil on the middle leg of said element, a source ot' magneto-motive-force in each ot' the partial magnetic circuits including one of the other legs, a thermionic amnliiier connected to said coil and adapted to be influenced hy voltages induced in said coil, and an electrfi-responsive device governed hy said thermionic amplifier.

23. A car-carried element Vtor automatic train control systems comprising, a threelegged yoke. noi-mall;v energized primary coils on two ot said legs, a secondary coil on the third leg, each ot said two legs having an extending pole piece extending suhstantially the full length of saidyoke.

in testimony whercot1 I aiix my signature.

CHARLES S. BUSHNELL. 

